This invention relates to magnetic field change detection circuitry and more particularly to circuitry adapted to generate a signal that indicates passage of a ferromagnetic marker through a magnetic field.
As is known in the art, magnetic field sensors have been used to detect passage of a ferromagnetic marker through a magnetic field. The magnetic field sensor produces an output related to the strength of the magnetic field passing through the sensor. One type of magnetic field sensor is a Hall effect cell. Thus, if the Hall effect cell is mounted to the marker and the marker moves relative to a magnet which produces a fixed magnetic field, the output of the Hall effect cell provides a measure of the relative position between the Hall effect cell and the magnet. Thus, in one application, the Hall effect cell may be fixed to an automobile engine and coupled to ignition timing circuity for the engine while the magnet is affixed to the engine crank shaft. Thus, each time the crank shaft is properly positioned relative to the Hall effect cell, a timing pulse is produced by the Hall effect cell for the ignition system.
In another application, a gear is affixed to the crank shaft. The gear has disposed along its outer periphery a ferromagnetic tooth with edges terminating in a notch. The magnet and the Hall effect cell are fixed to the engine with the cell disposed between the magnet and the gear. The magnetic field is coupled to the Hall effect cell through the peripheral portion the gear. Thus, the strength of the magnetic field coupled to the Hall effect cell is a function of the relative position between the cell and the tooth. In one application, two Hall effect cells are used. The Hall effect cells are laterally spaced from each other along a direction of the path of tooth through the magnetic field. Thus, as the tooth passes one of the cells, the voltage produced by such cell increases, rises to some level and then decreases. The same voltage is produced by the other Hall cell, albeit delayed in time by T=d/v, where d is the lateral distance between the cells and v is the velocity of the tooth passing by the cells. The two signals produced by the cells are fed to a differencing circuit. Thus, the differencing circuit produces a pulse when the tooth edge passes between the pair of cells.